cmd/5g, cmd/6g, cmd/8g: provide embedded trampolines with argument size information

An embedded trampoline is a function that exists to marshal
a receiver of type *S to a receiver of type *T when T is an
embedded field in S.

Embedded trampolines are generated by a special path through
the compiler and are not subject to the general analysis and
annotation done to functions.  Their effects must be provided
explicitly.

R=golang-dev, r, daniel.morsing, minux.ma
CC=golang-dev
https://golang.org/cl/9874043

src/cmd/5g/gobj.c

@@ -548,8 +548,9 @@ out:
 	p->from.name = D_EXTERN;
 	p->from.sym = newnam;
 	p->to.type = D_CONST2;
-	p->reg = 7;
-	p->to.offset2 = 0;
+	p->to.offset = 0;  // stack size
+	p->to.offset2 = rnd(method->type->argwid, widthptr);  // argument size
+	p->reg = 7;  // textflag
 	p->to.reg = NREG;
 //print("1. %P\n", p);
 

src/cmd/6g/gobj.c

@@ -528,8 +528,9 @@ out:
 	p->from.type = D_EXTERN;
 	p->from.sym = newnam;
 	p->to.type = D_CONST;
-	p->to.offset = 0;
-	p->from.scale = 7;
+	p->to.offset = 0; // stack size
+	p->to.offset |= rnd(method->type->argwid, widthptr) << 32;  // argument size
+	p->from.scale = 7;  // textflag
 //print("1. %P\n", p);
 
 	mov = AMOVQ;

src/cmd/8g/gobj.c

@@ -534,8 +534,9 @@ out:
 	p->from.type = D_EXTERN;
 	p->from.sym = newnam;
 	p->to.type = D_CONST;
-	p->to.offset = 0;
-	p->from.scale = 7;
+	p->to.offset = 0;  // stack skize
+	p->to.offset2 = rnd(method->type->argwid, widthptr);  // argument size
+	p->from.scale = 7;  // textflag
 //print("1. %P\n", p);
 
 	mov = AMOVL;